Germanium monosulfide or Germanium(II) sulfide is the chemical compound with the formula GeS. Germanium sulfide is a red-brown powder or black crystals. It hydrolyzes slowly in moist air but rapidly reacts in water forming Ge(OH)<sub>2</sub> and then GeO. It is one of a few sulfides that can be sublimed under vacuum without decomposition. It also forms chalcogenide glasses and is a semiconductor.
First made by Winkler by reducing GeS<sub>2</sub> with Ge. Other methods include reduction in a stream of H<sub>2</sub> gas, or with an excess of H<sub>3</sub>PO<sub>2</sub> followed by vacuum sublimation.
It has a layer structure similar to that of black phosphorus. The Ge-S distances range from 247 to 300 pm. Molecular GeS in the gas phase has a Ge-S bond length of 201.21 pm.